OsmAnd does not touch your sdcard when uninstalling (just like a photo app won't touch the photos you've taken with the app). So unless you're planning on developing for OsmAnd (and want to compare the two releases), there's no reason to keep the free OsmAnd. You can safely remove it without problems.
Do note that some features in the database of OsmAnd (like your favourites, or tracks that haven't been written to the sd card) will be removed when you delete the free OsmAnd.